Micromachined cantilevers are commonly used in such applications as atomic force microscopy (AFM), scanning probe microscopy (SPM), data storage and accelerometers. Conventional micromachined cantilevers consist of flat, horizontal beams that flex or deflect in response to forces applied at the tip. Thus, the tip can be scanned across a surface and the cantilever deflections measured in order to generate information about the topology of the surface.
The prior art teaches about cantilever structures for sensing forces acting directly on the tip of a cantilever. For example, Clabes et al. in U.S. Pat. No. 5,321,977 discuss how to mount a piezoelectric jacket consisting of four sensors on a tip stem. These sensors extend along the edges of the tip stem and respond to the bending of the stem during a scanning cycle. Since the amount by which each of the sensors is bent varies, the three dimensional position of the tip can be derived from the signals generated by the sensors. Clabes uses only a single stem for mounting all the piezoelectric sensors and does not use a multibeam structure.
Another method of detecting cantilever deflections is by piezoresistive sensing. In this approach, the cantilever is made of or incorporates a piezoresistive material. A piezoresistive material is a type of material whose electrical resistance changes with internal mechanical stress. Compression and tension will result in opposite polarities of change in electrical resistance. When a piezoresistive cantilever flexes, the stress in it changes, causing changes in its electrical resistance. By measuring this variation in electrical resistance, the deflection of the cantilever can be derived.
In U.S. Pat. No. 5,083,466 Holm-Kennedy et al. disclose a multibeam structure for measuring displacement of one or more response elements to detect multi-dimensional components of an applied force. The structures taught by the inventors include response elements mounted on flexible beams made of silicon. The bending of the beams are usually sensed by piezoelectric and capacitive methods. In one of the embodiments disclosed by Holm-Kennedy et al. ('466), the intrinsic piezoresistive quality of the beam material (silicon) is used for sensing the elongation and contraction of the beam. The geometry of the arrangement taught does not allow one to sense bending of the beam because for a piezoresistor to change resistance, it must experience net compressive or tensile stress. Since the piezoresistor described by Holm-Kennedy occupies the entire volume of the beam, any pure bending deformation will result in the mutual cancellation of compressive and tensile piezoresistive responses. A similar effect occurs if the piezoresistor is isolated to the central region of the beam. The piezoresistor must be asymmetrically located on or within the beam to detect bending.
Other types of micromachined devices do exist that allow the use of cantilever beams to detect bending motion with piezoresistors. One such device is disclosed by Albrecht et. al. in U.S. Pat. No. 5,345,815. In this approach, the piezoresistor is located close to one surface of the beam, so that when the beam bends, the piezoresistor senses a net compressive or tensile stress. Albrecht has only one axis of compliance, so it is limited to sensing bending in one direction-vertical. U.S. Pat. No. 5,444,244 to Kirk et al. describes a piezoresistive cantilever which can sense both vertical and lateral bending by means of vertical and torsional bending modes of the cantilever. In accordance with this solution two planar piezoresistive elements are mounted on a flat cantilever made up of two flat beams. Preferably, the flexibility of the beams is ameliorated and restricted to predetermined locations on the cantilever by corrugations or notches serving as flex points. The piezoresistors are placed on the beams preferably close to or at those flex points to maximize the piezoresistive effect. Lateral and vertical deflections of the tip translate into torsion and bending of the piezoresistive elements. The piezoresistive elements are arranged such that their fractional resistance changes are different depending on whether the deflection of the cantilever is lateral or vertical. The properties of such cantilever systems are described by J. Brugger et al. in "Lateral Force Measurements in a Scanning Force Microscope with Piezoresistive Sensors" presented at Transducers '95, 8th International Conference on Solid-State Sensors and Actuators and Eurosensors IX, Stockholm, Sweden, Jun. 25-29, 1995, pp. 636. This invention requires the use of signal processing circuitry to separate the vertical and lateral components of the applied force. The vertical deflection information is derived from the sum of the resistance change in the piezoresistive elements and the lateral deflection is similarly derived from the difference.
A structure similar to that taught by Kirk et al. is also found in U.S. Pat. No. 5,386,720 to Toda et al. The inventors discuss an integrated SPM sensor including a cantilever with two beams. The general construction of this cantilever is designed such that a displacement of the right portion of the cantilever or the left portion of the cantilever are measured relative to a central longitudinal axis. The torsion of the cantilever is detected on the basis of the detection signals from the detection means measuring the differential resistance change in the two cantilever beams.
The cantilevers produced according to these techniques are limited to flat structures. In addition, since the same mechanical element is used in two detection modes, the vertical stiffness of the cantilever has to be balanced against its torsional stiffness during the design process, leading to potential compromises in versatility and performance. Also, the mixing of signals associated with vertical and lateral forces in these devices can lead to the difficult problems of signal separation and data interpretation.